Proxy backups are generated to assist in diagnosing MSB. Its goal is to
create a small, emailable facsimile of your backup, altering the
contents to remove personal data.

This option is included in MSB 3.1b4 and later releases.
What is inside this proxy backup and how can I be sure?

The proxy is a file-for-file representation of your backup. Files
containing personal data are altered to remove the data. For a
technical explanation, see this page.

Making the Proxy Backup

Launch MSB and then press and hold the ALT key while startup completes.
You may see a beta notice: click Cancel but continue to hold the
ALT key until MSB displays the following prompt:

Select OK. Once prepared, MSB should reveal the proxy folder; it is
located alongside the normal 'Backup' folder. You should than use your
system's compression utility to create a 'zip' file before attaching to
email.